The present invention relates to gas sensing instruments, and more specifically to compensating gas sensors for the effects of ambient conditions such as temperature and humidity. The present invention also relates to metal oxide semiconductor sensors utilized as gas sensors.
For safety purposes, gas-sensing instruments are used in many industrial applications such as in fuel cell systems whose feedstocks are flammable gases. It is well known that many gas sensorsxe2x80x94metal oxide semiconductor (MOS) based sensors in particularxe2x80x94suffer from environmental dependencies. That is, ambient temperature and relative humidity substantially affect their sensitivity. For example, one commercially available MOS sensor model is the Figaro TGS821 hydrogen sensor. Due to the combination of this sensor""s environmental dependencies and the environmental uncertainties to which it will be exposed to in certain fuel cell applications, a sensor reporting a reading of 526 PPM of hydrogen might actually be exposed to a true concentration ranging between 182 and 1627 PPM. In certain fuel cell applications, the lower reading would be regarded as being well below alarm-level whereas the higher reading would be regarded as being well above. This 8.9:1 range of uncertainty is the source of much frustration with uncompensated MOS gas sensors.
Accordingly, many designers of gas sensing instruments elect to compensate for MOS gas sensors"" environmental dependencies. The conventional wisdom is that this requires a microprocessor, firmware (software), and lookup charts. However, dependence upon firmware being perpetually executed without error in a microprocessor-based circuit greatly complicates efforts to design a highly reliable, fail-safe gas-sensing instrument. Furthermore, the conventional method produces compensation factors that are inexact approximations of the required values.
Attention is invited to the following U.S. patents, which are incorporated herein by reference: U.S. Pat. Nos. 5,716,506 to Maclay et al.; 4,313,338 to Abe et al.; 4,801,211 to Yagi et al.; 6,126,311 to Schuh; and 5,969,231 to Qu et al.
U.S. Pat. No. 5,716,506 to Maclay et al. discloses (see Col. 1) a gas sensor that compensates for relative humidity and temperature of the air in the detection of a predetermined gas in a microfabricated electrochemical sensor.
U.S. Pat. No. 4,313,338 to Abe et al. relates to a gas sensing device comprising a resistive film formed of ultra fine particles of a metal oxide (Col. 4, lines 10-15). The gas sensing device includes (Col. 7, line 43-Col 8, line 65) a temperature sensing element for maintaining the temperature of the gas sensitive element constant. U.S. Pat. No. 4,313,338 also discloses obviating the problem of water vapor obstructing the successful measurement of the concentration of gas by using a single gas sensing element to sense both the concentration of water vapor and the concentration of isobutane gas (see Col. 8, line 47-Col. 9, line 11). The gas sensing element is heated up 300 degrees Celsius during the measurement of the concentration of the isobutane gas and is cooled down to the room temperature of 25 degrees C. during the measurement of relative humidity.
U.S. Pat. No. 4,801,211 to Yagi et al. discloses (see Abstract) a humidity sensor that, when temperature corrected, indicates a dew point at a fixed temperature. By adjusting this fixed temperature dew point output according to a sensed temperature, the dew point can be detected. FIG. 2 shows analog circuitry employed with same. The sensor is fabricated from metal oxide ceramic material (see Col. 4, lines 44-46).
U.S. Pat. No. 6,126,311 to Schuh discloses (see FIG. 4) a sensor that outputs dew point, ambient temperature, and relative humidity. This patent discloses (see Col. 1, lines 14-20) that the relative humidity and dew point of a gaseous sample are closely related by well known algorithms for converting dew point and ambient temperature to relative humidity or converting relative humidity and ambient temperature to dew point. This patent also indicates (see Col. 2, lines 19-23) that a group of prior art sensors measure the relative humidity of an ambient environment as opposed to dew point, and that relative humidity and dew point are easily converted from one to the other with a measurement of the ambient air temperature.
U.S. Pat. No. 5,969,231 to Qu et al. discloses a sensor for monitoring the concentration of moisture and gaseous substances in the air. Semiconductive metal oxides are used (see Col. 1).
Notwithstanding the prior art teachings noted above, none of these references singularly or in any permissible combination teach a simple approach for compensating gas sensor measurements for both humidity and temperature at the same time. It would be advantageous therefore, to be able to perform such compensation utilizing analog circuitry, which would be highly reliable and fail safe.
Gas sensors are used, in various industrial applications, such as in the fabrication of fuel cells. For example, gas sensors configured to sense hydrogen can be employed to detect hydrogen fuel leaks or hydrogen fuel flow in the fuel cells. In this regard, attention is directed to commonly assigned U.S. patent application Ser. No. 09/322,666 filed May 28, 1999, listing as inventors Fuglevand et al., and which is incorporated by reference herein. This application discloses the particulars of how gas sensors can be employed in one form of a fuel cell system.